1. Field of the Invention
This invention relates to a method and device for use in detecting a precise focussing point for the purpose of bringing the focus of a lens into precise conformity with a predetermined plane, and further to a specific automatic focus adjusting device.
2. Description of the Prior Art
As is well known, when the focussing information is imparted through a lens to a photoelectric element or cell, such as cadmium sulfide, placed on a predetermined plane on the optical axis or optical path of said lens, with said lens being shifted, then the internal resistance of said cadmium sulfide (which will be referred to as `CdS` hereinafter) will present the focussing characteristic curve as shown in FIG. 1 (A). In this respect, when the luminance of the photographic object varies, the total height of the focussing characteristic curve will vary therewith, whereas the position of the maximal point will remain unchanged. In FIG. 1 (A), shown at a point P on the curve is a precise focussing point of the object, at a point (a) the position displaced ahead of the precise focussing point and at a point (b) the position displaced rearwards thereof. To obtain the precise focussing point from the variation in the internal resistance of the CdS having such characteristics, the CdS is inserted in a D.C. circuit to thereby convert the variation in the internal resistance into that of an electric current, whereby the precise focussing point P is detected by means of the electric current thus obtained. However, there arise difficulties in that, due to the fact that in the internal resistance of the CdS at the vicinity of the precise focussing point P is very small there results a small variation in the electric current which would be obtained subsequent to such conversion to electric current, so that the detection of the precise focussing point becomes difficult. In addition, due to the small variation in the electric current, the variation thereof will fail to be maintained constant when subjected to the voltage or temperature variations in the circuits, thereby presenting inaccurate detection of the precise focussing point. To avoid those shortcomings, there has been proposed an attempt, in which two CdS photoelectric cells are placed on the optical axis at positions corresponding to those of the front and rear points (a) and (b) of FIG. (A), whereby to differentially detect the focussing information of the object, i.e., by obtaining the mid point between the points (a) and (b) where the internal resistances of the CdS photocells become equal to each other.
This presents no objectional matter in the case where the internal resistance of the CdS varies symmetrically with respect to the precise focussing point P, and the precise focussing point P can be detected according to the aforesaid differential means. However, there arises a possibility, on the other hand, of obtaining an inaccurate detection of the point P, because the internal resistance of CdS varies non-symmetrically as shown in FIG. 1 (B), with respect to the precise focussing point P, depending on the CdS used.